The preparation and characterization of a murine monoclonal antibody (McAb 33.1) has been completed. The tissue and cellular distribution of the 33.1 antigen suggests that it is a polymorphic human B-lymphocyte specific marker. Flow cytometric analysis of a panel of EBV transformed human homozygous typing cells suggests that it cosegregates with MB1 and MB3 positive cells. Limited radio-amino acid sequence studies suggest that it is a new non DR, but Ia-like antigen.